Laser-based remote phosphor lighting systems are an emerging trend, especially in the automobile industry but may soon be used in other applications for general illumination such as street lights for example. In these systems, a beam from a laser diode excites the phosphor, which in turn emits non-coherent light. One of the more critical limitations to widespread use of laser phosphor lighting systems is the safety of such systems when malfunction or physical damage occurs. For example, in an automobile crash, the laser beam might point away from the target (phosphor) into the open air. If using lasers in the range of 5 watts or more, this can be dangerous, causing a fire hazard and/or severe damage to the skin or the eyes of observers. This is one reason that the use of laser-driven headlights in the United States has been limited.
U.S. Pat. No. 8,400,011, entitled “Illumination Device, Automotive Lighting Equipment, and Vehicle”, issued on Mar. 19, 2013 to Kawaguchi et al. describes a system for sensing a malfunction in a laser-driven fluorescent lighting system. The system disclosed in the '011 patent uses a photodiode to detect the presence of reflected light at the wavelength of the laser. It discloses the use of a fluorescent plate having a reflective region within the region on the fluorescent plate that the laser beam is designed to irradiate. Under normal operation, a portion of the laser beam is reflected from the reflective region of the fluorescent plate, while the remaining portion of the laser beam excites the fluorescent phosphor. The purpose of the photodiode is to detect the presence of reflected light at the wavelength of the laser. However, if there is structural damage to the fluorescent plate and its reflective region, such as in an automobile crash, and the photodiode is not able to detect the presence of the reflected light at the wavelength of the laser, and the system will automatically shut down the laser diode for safety purposes. One drawback is that this safety system requires that the fluorescent plate be manufactured with a reflective region. In one embodiment, see FIG. 13 in the '011 patent, Kawaguchi et al. explain that the reflective region on the fluorescent plate may not be necessary because the fluorescent plate may itself sufficiently reflect the laser beam without the aid of a reflective region. In this embodiment, Kawaguchi et al. describe the use of an optical filter to block light illuminated from the fluorescent plate except for light at the wavelength corresponding to that emitted from the laser diode. One of the drawbacks of either system described in the '011 patent is the risk that a malfunction is not detected because the photodiode continues to detect light at the wavelength of the laser beam, even though there is structural damage to the luminaire. For example, after an automobile crash, detected light could come from another light source, or from the laser beam reflected from another damaged component. The present invention is designed to address these issues.